Process for preparing abrasion-resistant coated catalysts

ABSTRACT

A process is disclosed for preparing coated catalysts, inter alia, for gas phase oxidations in organic chemistry, comprised of an inert support and a coating of catalyst material enclosing this support, wherein a suspension of the starting material for the coating is sprayed onto an agitated bed of the support, while the suspending medium is being partially removed by a gas stream, and the raw material is then dried and heat-treated. For this purpose, the support bed is mechanically agitated and loosened by a gas stream blown in from below. The catalyst precursor containing a binder and, if appropriate, a pore-former is sprayed in an increasing amount from above onto the bed, the ratio between suspending medium sprayed on and drawn off remaining about constant. The thermal expansion coefficient of the precursor as a dry powder must not deviate by more than 15% from that of the support. The applied coating is densified by continuing the mechanical and fluidizing mixing motion, the material is then dried in a continuing gas stream and heat-treated, if appropriate after decomposition of an added pore-former.

This is a continuation of our copending application Ser. No. 390,446filed June 21, 1982, now abandoned, which is relied on and incorporatedherein by reference.

The invention relates to a process for preparing abrasion-resistantcoated catalysts formed of an inert support which has a rough surfaceand a particle size of 0.5 to 6 mm and a coating of active catalystmaterial enclosing the support and anchored in it, the process beingcarried out by agitating a bed of the support and spraying thereon asuspension of the starting material for the coating while partiallyremoving the suspending medium by a gas stream of 20°-250° C.,essentially constant residual moisture of the coating being maintained,and drying and heat-treating. The invention also relates to thecatalysts produced thereby and uses of these catalysts in selectedcatalytic reactions.

It is known to use for catalytic oxidations catalysts in which thecatalytically active constituents are arranged as a coating on an inertsupport which is in a particulate form or in some other shaped form.

This measure requires less of the expensive catalytically activecatalyst material per reaction volume, and less expensive catalysts canthus be prepared.

In addition, the catalytic properties of catalysts can also be improved.Namely, this arrangement of active substance on the support surface; onthe one hand, avoids local overheating phenomena, owing to thetemperature-equalizing effect of the support mass, and, on the otherhand, shortens the diffusion paths for gaseous reactants. Moreover, thisarrangement makes it possible, by applying various layer thicknesses,purposely to produce more or less active catalysts (Austrian Pat. No.226,672 corresponding to U.S. Pat. No. 3,232,977, the latter beingrelied on).

German Auslegeschrift No. 2,165,335 describes a process for preparingacrolein by oxidizing propylene with a gas containing molecular oxygen,in the gas phase at an elevated temperature in the presence of acatalyst, in which process a pulverulent composition containing, forexample, the elements MoBiCoNiFeBNaSnSiO adsorbed onto an inactive,porous support shape, such as α-Al₂ O₃. The material is here absorbed byapplying the wet-milled catalyst composition onto porous α-Al₂ O₃spheres of 5 mm diameter, followed by drying and heat-treatment.

German Offenlegungsschrfit No. 2,351,151 describes a process forpreparing a coated catalyst intended for oxidation, ammoxidation oroxidative dehydrogenation of an olefin, in the preparation of whichcatalyst an inert support having a diameter of at least 20 μm ispre-wetted with a liquid and dry pulverulent catalyst material is thenadded and the mixture is slowly stirred.

According to German Offenlegungsschrift No. 2,250,200, a coated catalystfor cleaning exit gases from motor vehicles and industrial plants can beobtained by producing a catalytically active coating on shapes composedof heat-resistant support material by thoroughly mixing them, with theuse of a liquid binder, with a pulverulent calcined active componentwhich preferably has particle dimensions below 100 μm and then removingthe binder by heating. Afterwards, the support core and the firmlyadhering coating are present without significant mutual penetration.

Finally, the process described in European Laid-Open Application No.0,015,569 involves, in the preparation of coated catalysts, applying anaqueous suspension of the catalytically active material onto agitatedsupport particles, the suspension being sprayed at a certain constantrate onto the support, while the suspending medium is being partiallyremoved by means of a gas stream of 20°-300° C., and an essentiallyconstant residual moisture of the coating being maintained. U.S. Pat.No. 4,305,843 corresponds to the above European application and isrelied on.

The catalysts which can be obtained by the known processes have thecommon disadvantage that in the case of thicker coatings, that is,coatings the amount by weight of which, relative to the catalyst,exceeds 20%, the abrasion resistance and impact strength of the coatingare not fully satisfactory for use in large scale industrial fixed bedreactors.

In particular, a tendency for the coating to spall under the influeneceof temperature gradients was found in the case of coated catalystsmanufactured by means of conventional coating pans or rotary disks,which only permit the passage of a drying gas stream across the agitatedmaterial.

Moreover, only a relatively wide particle size distribution, which isdetermined by the particular thickness of the coating of the individualparticles of the catalyst, can be obtained by means of these devices.

However, a wide particle size distribution results in, on the one hand,a markedly higher pressure drop of catalyst beds and, on the other hand,the occurrence of strongly differing heats of reaction on the individualcatalyst particles, which, in total, leads to deterioration ofselectivity.

The preparation process dealt with in European Laid-Open Application No.0,015,569 requires the maintanance of a metering rate for suspension anddrying gas which remains constant with time, in order to maintain thewater content pension of the resulting coating during the spraying-on ofthe suspension at a virtually constant value. However, it is preciselythis measure which, with increasing duration of the preparationoperation, causes the outer surface of the coating to containincreasingly less liquid, which impairs or prevents the application ofthicker coatings having sufficient mechanical strength.

Moreover, by guiding the dry gas stream over the surface of the supportbed, a measure proposed there, only a moderate drying rate is obtainedduring the formation of the coating. The result of this is theunfavorable wide particle size distribution already mentioned.

The object of the invention is to provide a process for preparing coatedcatalysts which imparts to the latter abrasion resistance,temperature-change resistance and a narrow particle size distribution atthe same time as good catalytic properties.

The invention relates to a process for preparing abrasion-resistantcoated catalysts comprised of an inert support which has a rough surfaceand a particle size of 0.5 to 6 mm and a coating of active catalystmaterial enclosing this support and anchored in it, by agitating a bedof the support and spraying thereon a suspension of the startingmaterial for the coating while partially removing the suspending mediumby a gas stream at a temperature of 20°-250° C., essentially constantresidual moisture of the coating being maintained, and drying andheat-treating, wherein a mixing motion is imparted to the support bed bymechanical action and the support bed is simultaneously loosened byblowing in from below a fluidizing, mixing-intensifying gas stream; asuspension of a precursor of the catalytically active material, whichsuspension contains a binder and, if appropriate, a pore-former, ispassed countercurrent to the gas to this bed at a rate which increaseswith increasing thickness of the coating, the amounts of suspendingmedium drawn off and sprayed being maintained in a substantiallyconstant ratio which is determined by the particular combination ofsupport and precursor used and the thermal expansion coefficients ofsupport and of dried pulverulent precursor being so chosen that theydiffer by at most 15%, and wherein after the spraying-on has beencompleted the coating is densified by continuing the increased mixingmotion, the mechanical mixing motion is then stopped, the material isdried in a continuing gas stream and finally heat-treated, ifappropriate after decomposition of an added pore-former.

The new coating process for support bodies proposes loosening up asupport bed set in mixing motion by blowing in a gas stream from below,the gas stream passing through the fluidized charge effecting partialremoval of the suspending medium. For carrying out this step,appropriately equipped mixing units are possible, such as, for example,special coating drums, coating pans or rotary disks. Those units arepreferable in which drying air streams evenly through the entire bed.

The use of a so-called Driacoater in a countercurrent method, in whichspray liquid and dry air flow in opposite directions has provedparticularly advantageous. This piece of equipment has been described,inter alia, in German Offenlegungsschrift No. 2,805,801, relied onherein, and it primarily comprises a cylindrically or conically shapedand horizontally mounted drum. Dry air is introduced exclusively frombelow the underside of the bed of material via air ducts located in theouter jacket of the drum through hollow ribs arranged on the inside ofthe drum and which are perforated on the side facing away from thedirection of rotation.

When the drum revolves, the bulge-shaped hollow ribs and the drying airblown in through them effect the fluidization and thorough circulationof the bed material; that the drying air flows evenly through the lattermanifests itself in a uniformly and calmly downflowing intrinsic motionof the material. Moisture-rich exit air is drawn off above the bed viathe hollow uptake mandrel in the axis of revolution of the drum.

For spraying the powder suspensions used in the process according to theinvention, two-material nozzles are preferably used, by means of which,more simply than in the case of one-material nozzles, the desired feedrate, with any state of fine division, can be conveniently controlled.

The atomization is usually effected by means of compressed air of 0.5-2bar, as a function of the necessary suspension throughput (which resultsfrom the size of charge, the desired thickness of the powder applicationand the time for preparation) by means of one or more nozzles of 2-4 mmdiameter for pressures of the suspension in front of the nozzle of 1-3bar.

For Driacoater units having charge capacities of 10-200 liter, it hasprovided advantageous to adjust the fluidizing gas stream to a specificflow rate of 15-50 Nm³ per hour per liter of support and to heat it to60°-100° C. Lower inlet air flow rates lead to markedly lower dryingrates, less even flow through the entire bed due to wall effects alongthe drum walls and hence considerably longer preparation times. Incontrast, inlet air flow rates which are too high cause too severedrying of the suspension on the way from the nozzle to the bed surface,which causes the resulting dried precursor powder to be carried awaywith the exit air and inadequate moisture of the coating duringapplication. It has been found that maintaining a constant moisturelevel of the resulting coating during the entire coating build-up is anessential prerequisite for obtaining a firmly adhering coating of activecatalyst material firmly anchored in the support material. If thecoating of the blanks during this coating build-up becomes too moist,several particles agglomerate with one another. However, if applicationis too dry the desired anchoring in the support and also strength of thecoating cannot be obtained. It is also an essential insight that bymaintaining a drying air which is constant in respect of temperature andrate the necessary constant moisture of the coating can be readilycontrolled by the amount of suspension sprayed on per unit time.

The fluidizing gas may be air, nitrogen, or other inert gases. Drying ofthe gas is not necessary, but if the gas is not dry, it should havenearly constant humidity.

To assign nominal values for such a control, the temperature above thebed or the moisture of the exit air can be used, both of which permitsensitive monitoring of the drying process. The most favorable nominalvalues themselves depend on the type of powder and on the temperature,the moisture and the amount of inlet air per unit volume of supportmaterial. Depending on the solids content of the suspension and the typeof precursor, 10-50% of the sprayed-on suspending medium should remainin the coating during its build-up. It has been found that considerableimprovement of the mechanical stability of the coating is obtained ifnot a constant nominal value but a decreasing temperature or anincreasing exit air moisture are given. This makes possible, bycorresponding program control, a fully automatic application of theprecursor powder.

Water is preferably used as the suspending medium for the catalystprecursor present in powder form. Other liquids, such as, for example,alcohols, are not excluded and have in various points advantages overwater: they may require less vaporization energy or permit bettermatching of the wetting and solubility behavior of the precursor of thecatalytic material and the support substance. The latter can beinfluenced in the case of aqueous suspensions only by adding binders.However, the advantage of organic solvents is contrasted with thedisadvantage of forming ignitable mixtures with the drying air andrequiring special exit air cleaning units. The suspending medium shouldbe inert; that is, non-reactive during the course of the preparation.

The solids content of the suspension is best so adjusted that thesuspension is comprised of 20-80, preferably 40-70, % by weight, inparticular 55% by weight, or pulverulent precursor.

Solids contents which are too high can cause blockages in the feed andspray system for the suspension. Solids contents which are too low,however, require unnecessarily prolonged preparation times. Theempirically determinable solids content which is most favorable in aparticular case depends on the properties of the precursor used and itsinteraction with the suspending medium and, in the case of the catalystsprepared within the scope of the examples, is 55% for the propeneoxidation.

It has also been found that it is possible to obtain a markedimprovement in the abrasion resistance of supported catalysts by the useof binders as known from granulation. Their content in the suspensiondepends on the type of binder and is as a rule between 0.5 and 10%.While the lower limit is fluid andnthe minimum amount necessary toensure improvment of the abrasion resistance, in the case of binderconcentrations which are too high the drying rate during the preparationof the coating is frequently reduced. For the precursors of the activecatalyst component which were used, the best results were obtained with2-5%, in particular about 4%, by weight of glucose or urea.

Other binders include starch, sugar, sorbite, gum arabicum, propyleneglycol, stearic acid, oleic acid and glycerol. The function of thebinder is to tackify the surface of the carrier and the surface of theprecursor.

In certain reactions, such as, for example, the oxidation of propene toacrolein, a retardation of the reaction due to pore diffusion isobserved in particular when using coated catalysts having a highproportion of active phase, that is thick coatings. It has now beenfound that the addition of finely divided pore-formers sparingly solublein the suspending medium such as pentaerythritol, polymethylmethacrylate, polystyrene, polyvinyl alcohols or the like, can reducethis retarding influence on the reaction by the formation of macropores.

The pore-former enhances transport of reacting molecules within thecatalyst system. It may be a polymer or a monomer and should be onlyslightly soluble in the suspending medium. The pore-former must becapable of being burned off during the tempering step.

The preferable content in the suspension of pore-former is 1-10% byweight. It is a prerequisite for the action of the pore-former that itcan be removed again below the heat-treatment by thermolysis oroxidation of the built-up coating.

The invention explicity proposes using, for the build-up of the coating,a precursor of the catalytically active material. The term "precursor"is to be understood as meaning that the precursor material alreadycontains all the ingredients required for producing the completecatalytically active material by a subsequent specific heat treatment.

The precursor is a preformed catalytic material in powder which may be adried coprecipitate or a coprecipitate which has been heat-treated belowthe temperature of the final tempering step. As shown in the examples,the precursor may be oxidic or hydroxidic and is made from saltsolutions.

In the process according to the invention, preferably a coprecipitatefrom combined salt solutions of catalytically active elements, whichcoprecipitate is dried or has been calcined below the heat-treatmenttemperature, is used as the precursor of the catalytically activematerial.

The composition of this coprecipitate and its particular preparation isnot specific to the process according to the invention, but depends onthe desired catalytic action in the reaction where the coated catalystis used. Usually, the precursor can be prepared analogously to knownunsupported catalysts. To obtain good suspendability of the precursor inthe suspending medium and a trouble-free feed of the suspension, aparticle size distribution of 1-150 μm, having a maximum preferablywithin the range of 1.5-30 μm has proved advantageous.

The process according to the invention makes possible the preparation ofcoated catalysts in which the amount of the pulverulent precursor is0.1-2 times the weight of the support, this range not resulting fromspecific limits of the preparation process but rather from practicalconsiderations concerning the use of the catalysts according to theinvention. This means that in principle even those compositions can beprepared by the process according to the invention which are outside therange indicated.

The invention also explicitly proposes that the thermal expansioncoefficients of support and precursor are to be so adjusted that theysubstantially agree and differ at most by not more than 15%. For ifthese coefficients differ by more, the coating will crack in thesubsequent heat treatment step.

These cracks can become so large that flaky spalling of the coatingtakes place. In any case, the occurrence of cracks is associated with asharp reduction of the mechanical stability of the coating, that is theabrasion resistance. It has been found that matching of the thermalexpansion coefficients by selecting a suitable support is only possiblein some cases and is seldom adequate, since possible inert supports areall within the relatively narrow range from 50-90×10⁻⁷ /°C. (forone-dimensional expansion).

It has now been found, surprisingly, that the thermal expansioncoefficient of the precursor powder can be matched to the coefficient ofthe support by a heat pretreatment at 250°-600° C. The particularprecise conditions depend on the composition of the precursor and on thesupport to be used. Care must be taken here that this matching is to becarried out not for a certain temperature but for the entire temperaturerange of the subsequent heat treatment (the tensions between coating andsupport which occur in this heat treatment are responsible for possiblecrack formation). This means that an exact matching, which wouldpresuppose a firmly defined reference temperature, is not possible. Thisis particularly due to the fact that, in the materials to be usedaccording to the invention, different temperature dependencies of theexpansion coefficients are usually given for precursor and support.

Within the scope of the invention, the preparation of improved coatedcatalysts for four important gas reactions of organic chemistry whichuse heterogeneous catalysis is to be particularly emphasized, since itis precisely these processes which can be considerably improved by meansof these catalysts.

These reactions are the catalytic oxidation of propylene or isobutene toacrolein or methacrolein respectively, the catalytic gas phase oxidationof acrolein and methacrolein to acrylic acid and methacrylic acidrespectively, the catalytic gas phase oxidation of methanol toformaldehyde and the ammoxidation of aliphatic and aromtic hydrocarbonsto nitriles. For each of these reactions, the preparation of a suitablecoated catalyst using a precursor material which is already known initself and which is only in need of the final heat treatment determiningthe catalytic properties will be indicated below.

According to a preferable embodiment of the invention, the precursor fora coated catalyst for preparing acrolein or methacrolein from propyleneor isobutene accordingly used is an oxidic powder of the composition:

    Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d P.sub.e Mo.sub.f O.sub.x

in which a is a number from 2-20, b is a number from 0-15, a and b are anumber from 2-20, c is a number from 0.1-7, d is a number from 0.1-4, eis a number from 0.1-4, f is about 12 and x is a number from 35-85, and0.2 to 5% of tantalum or samarium, calculated as Ta₂ O₅ or Sm₂ O₃, and,if appropriate, also 0.05 to 3.0% of an alkali metal or alkaline earthmetal, calculated as oxide, if appropriate on a support substancecomposed of a layer lattice silicate and/or highly dispersed silica--inthe first case (that is, when the silicate plus silica are used) in aweight ratio of 10:1 to 1:1--are additionally used, and the coatedcatalyst is heat-treated for 0.05-5 hours at 520°-650° C. When an alkalimetal or alkaline earth metal is used, the elements K, Na and Mg arepreferable. Modification of these catalyts by alkali metals and alkalineearth metals is known in the art.

Layer lattice silicate is a silicate with a leaf structure and is easilycleaved along the crystal lattice network. Examples of such substancesare montmorillonite, talc, and kaolinite. High dispersion silicas aremade by flame hydrolysis of halosilanes, such as SiCl₄. These aresometimes called pyrogenic silica. Examples are Aerosil and Cab-O-Sil.

According to a further preferable embodiment of the invention, theprecursor used for a coated catalyst for preparing acrylic acid andmethacrylic acid from acrolein and methacrolein respectively is anoxidic powder of the composition:

    Sb.sub.1-60 Mo.sub.12 V.sub.0.5 -25W.sub.0.1-12 M.sub.0-12 O.sub.x

in which M denotes at least one of the elements lead, silver, copper,tin, titanium or bismuth and the coated catalyst is heat-treated for0.05-5 hours at 320°-450° C.

Moreover, it is possible for the precursor used for a coated catalystfor the oxidation of methanol to formaldehyde advantageously to be anoxidic powder composed of molybdenum and iron in an MoO₃ :Fe₂ O₃ ratioof 10 and which may have admixed 3-60% by weight of TiO₂ and the coatedcatalyst to be heat-treated for 3-10 hours at 300°-500° C.

Finally, the precursor used for a coated catalyst for the ammoxidationof alkyl-substituted aromatic and heteroaromatic hydrocarbons can be anoxidic powder composed of the oxides of antimony and vanadium in a ratioof 1.1:1 to 50:1 and which additionally contains at least one of theelements iron, copper, titanium, cobalt, manganese and nickel and, ifappropriate, support substance composed of layer lattice silicate andhighly dispersed silica, and the resulting coated catalyst isheat-treated for 2-8 hours between 600° and 1,100° C.

Suitable support materials for the coated catalysts obtainable accordingto the invention and which can be advantageously used in these and alsoin other reactions have proved to be, in particular, α-alumina, aluminumsilicate, magnesium silicate or silicon carbide. These are inert underthenconditions of the reaction described herein. As regards the shape ofthe support, the process has no special demands, but spherical supportsare preferable.

Nonporous or slightly porous magnesium silicate or silicon carbide isused above all when it is intended to apply the active phase only to thesurface of the support and not to introduce the phase into the cavitiesof the support. In contrast, the catalytic material is more stronglyprotected and better anchored in the cavities of macroporous α-aluminasand alumosilicates and, in coatings which are not too thick (less than20% by weight of active phase), requires a coating which is not so hard.The macropores of aluminum silicates and α-alumina should be within therange of 2-2,000, preferably 20-300, μm (90% value), in order, on theone hand, to ensure adequate strength of the support but, on the otherhand, to permit the depositing of active phase in the pores.

From the point of view of a favorable behavior during coating build-upslightly porous or nonporous supports have advantages, since, in thecase of these materials, a lower liquid loading of the support takesplace at the start of the preparation and the moisture leaving the poresat the end of the preparation in the drying process is more difficult tocontrol in the case of macroporous supports.

The invention also proposes that the support material should have arough external surface because this increases the bond strength of thecoating by in-depth anchoring of the catalytically active material inthe support and permits uniform application to the entire supportsurface. In the case of smooth support material surfaces, a flaky,irregular, thick application is usually observed. It has been found tobe particularly advantageous if the support surface has a roughness,characterized by the middle roughness value according to DIN 4,768/1,measured by means of the Hommel roughness meter of 5-50 μm.

The invention, in addition to describing the improved preparationprocesses for coated catalysts, also relates to the four uses of thesecoated catalysts already mentioned and to the catalysts per se.

The invention is illustrated in more detail below by means ofillustrative embodiments:

EXAMPLE 1

The coprecipitate for the preparation of the active catalyst phase wasprepared in a manner known from German Pat. No. 2,049,583, relied onherein, by successively adding with stirring a solution of 0.3 kg ofsamarium oxide Sm₂ O₃ in 3.5 kg of 38% strength nitric acid, 5.8 kg ofpyrogenic silica, Aerosil®, 10.8 kg of montmorillonite, a solution of23.4 kg of ammonium molybdate (NH₄)₆ Mo₇ O₂₄.4H₂ O in 31.4 kg of 3.5%strength phosphoric acid and a solution of 5.4 kg of bismuth nitrateBi(NO₃)₃.5H₂ O in 4.5 kg of 7.7% strength nitric acid to a solution of32.3 kg of nickel nitrate Ni(NO₃)₂.6H₂ O, 1 kg of cobalt nitrateCo(NO₃)₂.6H₂ O and 4.5 kg of ferric nitrate Fe(NO₃)₃.9H₂ O in 38 kg ofwater. The resulting suspension of the coprecipitate was dried on a drumdryer, calcined at 530° C. in a revolving tube and then milled. Theresulting powder of the precursor of the catalytically active materialhad a particle size distribution of 2-40 μm (>90%, maximum amount 15 μm)and, at 400° C., a thermal expansion coefficient of 81×10⁻⁷ /°C.

By suspending 6.5 kg of this precursor powder in 4.7 kg of water withthe addition of 0.5 kg of D-glucose as binder and 0.3 kg ofpentaerythritol (type R, made by Degussa) as pore-former, the suspensionfor the starting material of the coating was prepared. The supportschosen for this precursor material were fired steatite spheres whichhave a diameter of 4 mm, are virtually nonporous and have a roughsurface (middle roughness value 25 μm according to DIN No. 4,768/1) andthe longitudinal thermal expansion coefficient which is 90×10⁻⁷ /°C. at400° C.

6 kg of this support were introduced into a Driacoater 500 and given avigorous mixing and flowing motion in this unit by blowing in 2 m³/minute of preheated air at 80° C. and revolving the drum at 20 rpm.Then 0.4 liter of the suspension was sprayed in 2 minutes by means of atwo-material nozzle onto the support thus agitated. The spraying-on ofthe remaining suspension was controlled via the exit air temperaturefrom the pan in such a manner that all the time a constant moisture ofthe coating was observed. In this stage, the exit air temperature fellfrom initially 48° C. to 39° C. at the end of the application of thesuspension (after 60 minutes), and the rate at which the suspension wasapplied increased from 0.096 to 0.104 liter per minute.

At the end of the spraying-on process, there followed, while the drumcontinued to revolve, a densification phase of 5 nminutes and then a 20minute drying phase at a single pan revolution per minute.

After air drying overnight, the pore-former was decomposed in therevolving tube at 400° C. and a mean residence time of 15 minutes. Thecatalyst was activated at 550° and 15 minutes residence time, likewisein the revolving tube.

The coated catalyst obtained had a hard, crack-free coating. The meandiameter of the coated catalyst obtained was 5.25 mm with a standarddeviation of 0.3 mm. Abrasion was determined as amount of abradedmaterial smaller than 2 mm after 7 minutes in a La-Roche Friabilator byrolling and falling wear at 20 rpm and it was less than 0.2% by weightfor the heat-treated coated catalyst. After a heat treatment of 100cycles of heating-up and cooling-down, in which the catalyst was heated,in each case in 0.5 hour, from 250° C. to 400° C. and then cooled downagain to 250° C., the value had not significantly increased and was 0.2%by weight.

In a falling test, free fall of 100 ml of catalyst onto a hard surfacethrough a 3.4 m long tube having an internal diameter of 20 mm, theproportion of broken material of <2 mm produced was 0.03% by weight.

COMPARATIVE EXAMPLE 1

The precursor powder was prepared as in Example 1, only with thedifference that the drum-dried coprecipitate was calcined at 410° C. inthe revolving tube. The thermal expansion coefficient of the powder wasthen 50×10⁻⁷ /°C. at 400° C.

A coated catalyst was prepared analogously to Example 1 by means of thisprecursor powder. In the two revolving tube processes, the decompositionof pore-former and the heat treatment, strong abrasion took place (about5% by weight). The coating was severely cracked and, in places, pieceshad spalled off. For this catalyst, for which the thermal expansioncoefficient of the precursor of the support by a suitable heattreatment, abrasion in a La-Roche Friabilator was 15% by weight.

COMPARATIVE EXAMPLE 2

The precursor powder prepared according to Example 1 was used forpreparing a catalyst in a coating pan. For this purpose, 30 kg of thesteatite support used in Example 1 were initially introduced into the 50kg coating pan and given a mixing motion by a rotation of the pan at 21rpm at a pan inclination of 20°. Then 31 kg of the precursor powder weresuspended together with 2.5 kg of D-glucose and 1.5 kg ofpentaerythritol in 22 liters of water. The surface of the agitated bedwas impinged with heated air at 90° C. with a rate of 200 m³ per hour.To apply the coating, the suspension was sprayed on through atwo-material nozzle at a flow rate falling gently from initially 0.5liter of suspension per minute to 0.48 liter per minute after one hour.When application was complete (about 80 mintes), the coating wasdensified for a further 10 minutes in the pan which continued to run.The resulting coated catalyst was dried for 15 hours at 40° C., thepore-former was decomposed at 400° C. in the revolving tube, and a heattreatment was carried out in the revolving tube at 550° C. and 15minutes residence time.

The resulting catalyst had the following physical properties, the meandiameter was 5.3 mm with a standard deviation of 0.68 mm.

The rolling and falling wear in a La-Roche Friabilator (20 rpm, 7minutes running time) was 1% by weight before and 1.2% by weight after atemperature change stress treatment between 250° and 400° C. (100 cyclesin 50 hours). In a falling test of 100 ml of catalyst through a 3.4 mlong tube having an internal diameter of 20 mm, the proportion of brokenmaterial of <2 mm produced was 0.2% by weight.

EXAMPLE 2

The catalytic effect of the catalyst prepared in Example 1 was tested inan industrial reactor tube having an internal diameter of 20.5 mm andbeing externally cooled by a salt bath, with a catalyst bed depth of 2.7m by means of the conversion of propene to acrolein.

(a) Feeds of 5 moles of propene per hour, 40 moles of air per hour and10.1 moles of H₂ O per hour produced, at a salt bath temperature of 351°C., a conversion of 94%, an acrolein starting yield of 79.2 and a totalselectivity for acrolein and acrylic acid of 92.5%.

(b) Feeds of 5 moles of propene per hour, 38 moles of air per hour and29 moles of recycled exit gas per hour (composition: 7% of O₂, 1% ofpropene and 92% of inert gas; e.g. propane, nitrogen, carbon dioxide andwater) produced, at a salt bath temperature of 355° C., a conversion of94.9%, an acrolein yield of 79.5% and a selectivity for acrolein andacrylic acid of 92%.

EXAMPLE 3

A raw catalyst powder corresponding to German Pat. No. 2,145,851 wasused as precursor of the catalytically active material. This powder wassubjected for 8 hours at 300° C. to a heat treatment. It had acomposition of 67.1% by weight of MoO₃, 12.8% by weight of Fe₂ O₃ and20.1% by weight of TiO₂. The main range (90%) of the particle sizedistribution was between 1 and 10 μm, with the 50% value at 1.7 μm. Thethermal conductivity of the catalytic material was 73×10⁻⁷ /°C.

2 kg of this powder were suspended in 2 kg of water after the additionof 0.12 kg of urea (as binder). 6 kg of aluminum silicate supportshaving a specific surface area of less than 1 m² /g, a macroporositywhere 90% of the pores where between 30 and 250 μm, a surface roughnessaccording to DIN No. 4,768/1 with a middle roughness value of 40 μm, adiameter of 48 mm and a thermal expansion coefficient of 69×10⁻⁷ /°C.(at 400° C.) were introduced into a Driacoater 500 as supports for thisprecursor material. The support was given a thorough mixing and flowingmotion by blowing in preheated air at 95° C. at a rate of 4 m³ perminute and revolving the drum at 20 rpm.

The suspension of the precursor was sprayed in the course of 75 minutesonto the fluidized support in such a way that the exit air temperaturesank from initially 50° to 44° C. at the end of the application stage.After a further densification phase of 5 minutes in the Driacoater whilefurther fluidizing and drying the raw coated catalyst, the latter wasair-dried for about 15 hours and then heat-treated for 5 hours at 425°C. in an air stream. The abrasion resistance in the La-Roche Friabilatorstandard test (7 minutes, 20 rpm) was 0.3% by weight.

5,180 g (about 3,760 ml) of the finished coated catalyst were evenlypacked into nine tubes (internal diameter 18.1 mm) of a tube bundlereactor. The bed depth of catalyst was about 173 cm in the tubes, someof which were equipped with laterally introduced temperature sensors.

The tube bundle reactor was cooled by a circulated stream of moltensalt. The salt bath temperature was 301° C. A preheated gas stream atabout 290°-300° C. was fed at a rate of 4,640 liters (S.T.P.) per hourinto the reactor and had the following composition: 11.1% by volume ofmethanol and 12.8% by volume of oxygen with the rest being inert gases,mainly nitrogen in addition to small amounts of steam (about 0.5% byvolume). The maximum temperature in the catalyst bed was 355° C. The gasleaving the reactor was immediately cooled down; the condensableproducts were then absorbed into water. A yield of 93.1% offormaldehyde, relative to the amount of methanol used, was obtained at aconversion of 99% of the methanol used over an accounting interval of 72hours.

EXAMPLE 4

A precursor powder corresponding to Example 1 of German Pat. No.2,009,172, relied on herein, and containing antimony, molybdenum,vanadium and tungsten in a molar ratio of 6:12:3:1.2 was prepared. Thedrum-dried coprecipitate was largely converted into the oxides bycalcination at 250° C. in a revolving tube and then milled. The powderthen had a particle size distribution with a main range (>90%) of 2-50μm with a maximum at 4.7 μm and a thermal expansion coefficient at 400°C. of 86×10⁻⁷ /°C.

6.5 kg of this precursor powder were suspended in 3.5 kg of watertogether with 0.2 kg of glucose as binder and this suspension wassprayed on in the course of 75 minutes onto 6 kg of steatite support (asin Example 1) in a Driacoater.

During this step, the support was given a thorough flowing and mixingmotion by preheated air at 80° C. and rotation of the pan. Theactivating, final heat treatment took place at 360° C. in a revolvingtube with a residence time at 15 minutes. The abrasion resistance in theLa-Roche Friabilator standard test (7 minutes, 20 rpm) was 0.05% byweight.

58 g of this coated catalyst were packed into a reactor tube having aninternal diameter of 16 mm and being externally cooled by a salt melt. Agas stream comprising 1.4 moles of air per hour, 0.5 mole of water perhour and 0.16 mole of acrolein per hour was passed over the catalyst ata salt bath temperature of 301° C., and a conversion of 98.8% and anacrylic acid yield of 94.5%, relative to acrolein used, were obtained.

EXAMPLE 5

A precursor powder was prepared in accordance with German Pat. No.2,009,172 by coprecipitation of 23.3 kg of antimony trioxide, 4.7 kg ofammonium metavanadate, 12.8 kg of titanium dioxide, 11.7 kg ofmontmorillonite and 5.8 kg of pyrogenic silica, drum drying and 0.3hour's heat treatment at 450° C. The resulting powder, after milling,had a particle size spectrum of 1-20 μm (90%) with a maximum of 15 μmand a thermal expansion coefficient at 400° C. of 65×10⁷ /°C.

9 kg of this precursor powder were suspended in 6 kg of water togetherwith 0.4 kg of glucose as binder and 0.6 kg of pentaerythritol, andsprayed, in a Driacoater, in 85 minutes onto 6 kg of aluminum silicatespheres (as Example 3). In this step, the support was fluidized bypreheated air at 80° C. and revolving the pan, and the spraying processwas controlled in such a way that the exit temperature from the pan waslowered from initially 47° C. to 37° C. at the end.

After air drying (15 hours), the coated catalyst was finallyheat-treated by being successively treated in a muffle furnace for 3hours at 550° C., 1 hour at 650° C. and 3 hours at 770° C. The abrasionof the finished coated catalyst, in the LaRoche Friabilator standardtest, was 0.1% by weight.

The catalyst was excellently suitable for the ammoxidation of aromaticand heteroaromatic hydrocarbons.

EXAMPLE 6

2 kg of the precursor powder prepared as in Example 1 were suspended in1.9 kg of water with the addition of 0.05 kg of glucose as binder. In aDriacoater 500, 6 kg of an aluminum silicate support having a specificsurface area of less than 1 m² /g, a diameter of 4.8 mm, a macroporositywhere 90% of the pores were between 70 and 500 μm, a surface roughnessaccording to DIN No. 4,768/1 with a middle roughness value of 48 μm anda thermal expansion coefficient at 400° C. of 72×10⁻⁷ /°C. were given athorough mixing and flowing motion by blowing in preheated air at 70° C.at a rate of 2 m³ /min and by turning the drum at 12 rpm, and thesuspension analogous to Example 1 was sprayed in the course of 35minutes onto the support thus agitated in such a way that the exit airtemperature sank from initially 43° C. to 38° C. After drying, the rawcatalyst was activated at 575° C. in a revolving tube. The abrasion,measured in a La-Roche Friabilator, was 0.2% by weight.

EXAMPLE 7

A precursor powder was prepared according to Example 1, only with thedifference that 0.4 kg of potassium nitrate was additionally added tothe samarium oxide solution. The precursor powder calcined at 470° C. ina revolving tube had a thermal expansion coefficient of 80×10⁻⁷ /°C.

9 kg of this precursor material were suspended in 5.3 kg of watertogether with 0.7 kg of pentaerythritol (pore former) and 0.8 kg ofglucose (binder) and the suspension was sprayed in a Driacoater onto 6kg of thoroughly agitated steatite supports (as in Example 1). In thisstep, the inlet air supplied at 2.5 m³ per minute was preheated to 85°C. and the suspension sprayed on in the course of 95 minutes was meteredat such a rate that the exit air temperature sank from 51° C. initiallydown to 42° C. After drying, decomposition of pore-former and binder at400° C. and activation at 550° C. in a revolving tube, the catalyst hadan abrasion of 0.3% by weight in a La-Roche Friabilator.

EXAMPLE 8

A precursor powder was prepared analogously to Example 1 by successivelyadding with stirring a solution of 18.4 kg of ammonium molybdate (NH₄)₆Mo₇ O₂₄.4H₂ O in 24.1 kg of 3.1% strength phosphoric acid, a solution of7 kg of bismuth nitrate Bi(NO₃)₃.5H₂ O in 7.0 kg of 0.8% strength nitricacid and 6 kg of pyrogenic silica (Aerosil®200) to a solution of 6.7 kgof nickel nitrate Ni(NO₃)₂.6H₂ O, 12.3 kg of cobalt nitrate Co(NO₃)₂.6H₂O and 6.9 kg of ferric nitrate Fe(NO₃)₃.9H₂ O in 30.4 kg of water. Theresulting coprecipitate was dried at 140° C. on a drum dryer andcalcined at 535° C. in a revolving tube and then milled in a pin mill.

The resulting powder had a particle size distribution of 5-80 μm (90%value) with a maximum at 30 μm and a thermal expansion coefficient of85×10⁻⁷ /°C.

An abrasion-resistant catalyst was prepared in a Driacoater 500 in amanner corresponding to Example 1 from 7.5 kg of this precursor powderwith an addition of 0.6 kg of glucose and 0.5 kg of pentaerythritol in6.2 kg of water and 6 kg of steatite support. The abrasion was 0.25% byweight in a La-Roche Friabilator.

EXAMPLE 9

A precursor powder was prepared as in Example 8, but with the additionof 0.2 kg of KNO₃ to the first solution. The resulting powder had aparticle size distribution of 3-70 μm (90% value) with a maximum at 25μm and a thermal expansion coefficient of 84×10⁻⁷ /°C.

An abrasion-resistant coated catalyst was prepared in a Driacoater 500in a manner corresponding to Example 1 from 5.5 kg of this precursorpowder with 0.4 kg of glucose in 4.5 kg of water and 6 kg of steatitesupport. The abrasion was 0.3% bynweight in a La-Roche Friabilator.

EXAMPLE 10

50 ml of the catalyst prepared in Example 8 were packed into a tubereactor which had an internal diameter of 16 mm and was externallytemperature-controlled to 362° C. by a salt bath. Feeds, per hour, of0.25 mole of propene, 45 liters (S.T.P.) of air and 9.5 g of waterproduced a conversion of 92.5%, an acrolein yield, relation to propeneused, of 80.5% and a total selectivity, relative to propene used, of95.8%.

EXAMPLE 11

50 ml of the catalyst prepared in Example 6 were packed into a reactorwhich had an internal diameter of 16 mm and was externallytemperature-controlled to 370° C. by a salt bath. Feeds, per hour, of0.15 mole of isobutene, 35 liters (S.T.P.) of air and 10.5 g of waterproduced a conversion of 91%, a methacrolein yield of 74.1%, relative toisobutene fed in, and a total yield of methacrolein and methacrylic acidof 82.4%.

EXAMPLE 12

50 ml of the catalyst prepared in Example 7 were packed into a reactorwhich had an internal diameter of 16 mm and was externallytemperature-controlled to 355° C. by a salt bath. Feeds, per hour, of0.15 mole of t-butanol, 35 liters (S.T.P.) of air and 10.5 g of waterproduced a conversion of 92.8%, a methacrolein yield of 75.2%, relativeto t-butanol fed in, and a total yield of methacrolein and methacrylicacid of 81.9%.

EXAMPLE 13

50 ml of the catalyst prepared in Example 9 were tested as in Example 11at a salt bath temperature of 382° C. The conversion was 93.6%, themethacrolein yield was 75.6%, relative to isobutene fed in, and thetotal selectivity for methacrolein and methacrylic acid was 82.9%.

EXAMPLE 14

80 ml of the catalyst prepared according to Example 5 were packed into areactor which had an internal diameter of 20.5 mm and was externallytemperature-controlled to 430° C. Feeds, per hour, of 0.12 mole ofβ-picoline, 80.5 liters (S.T.P.) of air, 16 liters (S.T.P.) of ammoniaand 19 g of water produced a conversion of 89.5% and a selectivity fornicotinoyl nitrile of 79%, relative to β-picoline used.

We claim:
 1. A process for preparing abrasion-resistant coatedparticulate catalysts formed of an inert particulate support which has arough surface and a particle size range of 0.5 to 6 mm and a coating ofcatalytically active material enclosing said inert support and anchoredto said support, said process comprisingproviding a bed of said supportlocated in a cylindrically or conically shaped horizontal zone having ahorizontal axis of rotation, agitating said bed and spraying onto saidbed a suspension of a pulverulent precursor for the catalytically activematerial in a suspending medium, while partially removing the suspendingmedium by a gas stream at a temperature of 20°-250° C., to therebydeposit a coating of the said pulverulent precursor on said supportwhile maintaining essentially constant residual moisture of the coatingduring the build up of the coating on the inert support, said agitatingbeing accomplished by imparting a mixing motion to the support bedlocated in said horizontal zone by mechanical action of rotation of saidaxis of rotation, simultaneously loosening and thereby agitating thesupport bed by blowing in from below a fluidizing, mixingintensifyinggas stream; and at the same time passing a suspension of the precursorfor the catalytically active material, which suspension contains abinder, countercurrent to the gas in this bed at a rate which increaseswith increasing thickness of the coating, the amounts of suspendingmedium being removed by said gas and the amount of suspending mediumbeing passed countercurrent to the gas being maintained in asubstantially constant ratio which is determined by the particularcombination of support and precursor used, the thermal expansioncoefficients of support and of dried pulverulent precursor being sochosen that they differ by at most 15%, and after completing the passingof the suspension in countercurrent flow with the gas, increasing thedensity of the coating being formed on said particulate support bycontinuing the increased mixing motion, thereafter stopping themechanical mixing motion upon reaching the desired density and dryingthe coated support in ancontinuing gas stream wherein the spraying ofthe said suspension and the drying of the coated support are carried outin said cylindrically or conically shaped horizontal zone having saidaxis of rotation, the said fluidizing, mixing-intensifying gas streambeing introduced exclusively from beneath said support bed through meansfor introducing said gas stream arranged on the outside in an outerjacket of said horizontal zone, and through perforated hollow ribsarranged on the inside of said horizontal zone, the perforations on thehollow ribs being oriented away from the direction of rotation of saidaxis of rotation, and finally heat-treating the coated support.
 2. Theprocess as claimed in claim 1, wherein said suspension further containsa pore-former and the final heat treatment decomposes the pore-former.3. The process as claimed in claim 1, wherein the fluidizing gas streamis adjusted to a specific flow rate of 15-50 Nm³ per hour per liter ofsupport.
 4. The process as claimed in claim 1, wherein the suspendingmedium used is water.
 5. The process as claimed in claim 1, wherein thesuspension is comprised of the pulverulent precursor to 20-80% byweight.
 6. The process as claimed in claim 1, wherein the suspension iscomprised of 40-70% by weight of pulverulent precursor.
 7. The processas claimed in claim 1, wherein the suspension contains as binder 0.5-10%by weight of glucose or urea.
 8. The process as claimed in claim 1,wherein the suspension contains 2-5% by weight of glucose or urea as abinder.
 9. The process as claimed in claim 1, wherein 1-10% by weight,relative to the weight of the starting material, of a pore-former whichis sparingly soluble and finely divided in the suspending medium andwhich can be removed below the heat-treatment by thermolysis oroxidation is added to the suspension of the pulverulent startingmaterial for the coating.
 10. The process as claimed in claim 1, whereina coprecipitate from combined salt solutions of catalytically activeelements, which coprecipitate is dried or has been calcined below theheat-treatment temperature, is used as the precursor of thecatalytically active material.
 11. The process as claimed in claim 1,wherein the precursor is a powder having a particle size distributionrange of 1-150 μm.
 12. The process as claimed in claim 1, wherein theprecursor is a powder having a particle size distribution within therange of 1.5-30 μm.
 13. The process as claimed in claim 1, wherein theamount of pulverulent precursor is 0.1-2 times the support weight. 14.The process as claimed in claim 1, wherein the thermal expansioncoefficient of the precursor powder is adjusted by a heat pre-treatmentat 250°-600° C. to that of the support.
 15. The process as claimed inclaim 1, wherein the precursor used is an oxidic powder of thecomposition Ni_(a) Co_(b) Fe_(c) Bi_(d) P_(e) Mo_(f) O_(x) in which a isa number from 2-20, b is a number from 0-15, a and b are a number from2-20, c is a number from 0.1-7, d is a number from 0.1-4, e is a numberfrom 0.1-4, f is about 12 and x is a number from 35-85, and 0.2 to 5% oftantalum or samarium, calculated as Ta₂ O₅ or Sm₂ O₃, and, optionally,also 0.05 to 3.0% of an alkali metal or alkaline earth metal, calculatedas oxide, and the support substance is composed of a layer latticesilicate and/or highly dispersed silica, wherein the weight ratio ofsilicate and silica is 10:1 to 1:1, and the coated catalyst isheat-treated for 0.05-5 hours at 520°-650° C.
 16. The process as claimedin claim 1, wherein the precursor used is an oxidic powder of thecomposition Sb₁ -60Mo₁₂ V₀.5-25 W₀.1-12 M₀₋₁₂ O_(x) in which M denotesat least one of the elements lead, silver, copper, tin, titanium orbismuth and the coated catalyst is heat-treated for 0.05-5 hours at320°-450° C.
 17. The process as claimed in claim 1, wherein theprecursor used is an oxidic powder composed of molybdenum and iron in anMoO₃ :Fe₂ O₃ ratio of 10 and which may have admixed 3-60% by weight ofTiO₂ and the coated catalyst is heat-treated for 3-10 hours at 300°-500°C.
 18. The process as claimed in claim 1, wherein the precursor used isan oxidic powder composed of the the oxides of antimony and vanadium ina ratio of 1.1:1 to 50:1 and which additionally contains at least one ofthe elements iron, copper, titanium, cobalt, manganese and nickel and,the support substance is composed of layer lattice silicate and highdispersed silica, and the resulting coated catalyst is heat-treated for2-8 hours between 600° and 1,100° C.
 19. The process as claimed in claim1, wherein support used is α-alumina, aluminum silicate, magnesiumsilicate or silicon carbide.
 20. The process as claimed in claim 19,wherein α-alumina and aluminum silicate have a porosity which is suchthat 90% of the pores are within the range of 2-2,000 μm.
 21. Theprocess as claimed in claim 19, wherein α-alumina and aluminum silicatehave a porosity which is such that 90% of the pores are within the rangeof 20-300 μm.
 22. The process as claimed in claim 19, wherein themagnesium silicate and silicon carbide are virtually nonporous.
 23. Theprocess as claimed in claim 19, wherein the roughness of the supportsurface has a middle roughness value of 5-50 μm according to DIN No.4,768/1, as measured with the Hommel roughness meter.
 24. A coatedcatalyst produced by the method as claimed in claim
 1. 25. A coatedcatalyst produced by the method as claimed in claim 15.